This invention relates to the field of recognition radars and specifically to the use of such radars with guided missiles.
Recognition radars are used as target seekers on missiles to detect and then lock-in on the targets so that the paths of the missiles can be set to intercept the targets. Target seekers must be small enough to fit within missiles and, ideally, add little weight to the missile. It is of course desirable to have target seekers that do not require costly equipment.
FIG. 1 shows a missile 1 containing target seeker 2. Target 3 is a tank. To recognize the tank as the desired target, target seeker 2 must discriminate that target from ground clutter, e.g., trees 4, and from false targets, e.g., truck 5, both of which are in the target seeker footprint 10.
Target recognition or discrimination typically involves two stages. First, the position and spatial characteristics of the reflectors in the target seeker's footprint are determined. Such reflectors can be objects, like trees or rocks, or they can be parts of an object, like the front, the back, and the turret of a tank. The second stage involves determining whether the relative positions of certain detected reflectors resemble the desired target.
Target seekers or conventional missiles must be able to recognize targets with a high degree of accuracy, typically with one-foot resolution. Target recognition must also be sufficiently fast to allow the target seeker to sweep the desired area and give the missile sufficient time to correct its flight path.
One attempt to use an FM/CW radar system as a target seeker is shown schematically in FIG. 2. Linearized transmitter 20 generates a linearly-modulated frequency signal shown by the solid line in FIG. 3. This signal is sent to antenna 30 and transmitted into the target seeker footprint. The reflections of the transmitted signal are received by antenna 40 and receive electronics 45 forms a reflected signal represented by the dotted line in FIG. 3.
For each "effective sweep period," mixer 50 determines the difference signal, shown as f.sub.diff in FIG. 3, between the received signal and the transmitted signal. As shown in FIG. 3, the "effective sweep period" or "sweep period" of an FM/CW target recognition system is equal to the total period of the transmitted signal minus the travel time for the signal to propagate to the footprint and back to the receiver, in this case antenna 40.
The difference signal f.sub.diff contains information about the reflectors in the target seeker footprint. Filter bank 60 extracts certain of that information. The number of filters in filter bank 60 depends on the desired resolution. For one foot resolution in a 500 foot footprint, filter bank 60 would require 500 filters. This can make the target seeker extremely large.
Profile recognizer 70 examines the outputs of the filters in filter bank 60 to determine whether certain of the reflectors in that footprint define a target.
The cost of the target seeker in FIG. 2 is very large because linearized transmitter 20 must have almost no non-linearity to achieve the needed resolution. For one foot resolution, the linearized transmitter sweep range must be 500 MHz, which is about the upper limit of contemporary linearized transmitters. If the target seeker is 5,000 feet from a target, then one foot resolution requires that the non-linearity of the linearizer be no more than 1/5000 or approximately 0.02% over the 500 MHz range. Such accuracy far exceeds the specifications of conventional linearized transmitters and the development of such transmitters necessary for the FM/CW target seeker in FIG. 1 will be very difficult and expensive.
One objective of this invention is a low-cost target-seeking radar that is sufficiently accurate for conventional missiles.
Another objective of this invention is a missile target-seeking radar which is very small.
It is also an objective of this invention to have a low-cost, small target seeking radar which has a great deal of resolution and which employs conventional linearized transmitters.
Yet another objective of this invention is to recognize targets very quickly to minimize sensitivity to circuit drifts and missile velocity while maintaining great accuracy.
Additional objectives and advantages of this invention appear in the following description, will be obvious from that description, or may be learned by practice of the invention. The objectives and advantages of this invention may be realized and obtained by the methods and apparatus in the appended claims.